The invention pertains to dental instruments. More particularly, the invention pertains to such instruments with large diameter, more comfortable handles.
A number of dental instruments, such as scalers, have been marketed with elongated plastic handles as an alternate to traditional metal handles. Light weight, relatively large diameter resin handles are preferred by some dental professionals as they tend to reduce hand fatigue. Larger diameter resin handles not only are comfortable to use, but their weight does not increase significantly due to lower density of the resin when compared to stainless steel, aluminum or brass.
Resin handles have advantages in that they usually exhibit lower manufacturing cost than is the case with stainless steel, aluminum, brass or ceramic handles. In addition, resin permits greater design latitude than is the case with metal.
Known resin handled scalers have a diameter on the order of 0.375-0.410 inches. Metal scalers have handles with diameters on the order of 0.270-0.375 inches.
Known resin handle scalers have however certain limitations. Many known resin handled instruments do not appear capable of passing the currently specified torque test of ISO 13397-1 issued Dec. 15, 1995. That test mandates that an applicable instrument must be able to pass an applied torque test of 35.4 inch-pounds (400 newton-centimeters) without damage or relative motion between the instrument""s operating tip and the respective resin handle. Some commercially available scalers with resin handles can tolerate only about 18 inch-pounds of applied torque without damage or relative motion between handle and tip.
One way to increase the amount of torque which the instrument can resist is to increase the force necessary to press fit the tip of the instrument into the handle. This technique, which increases the interference between mating surfaces, is not suitable for use in resin handled instruments.
A greater degree of torque resistance can be attained by using a tubular metal insert around which the resin handle is molded. The tip can then be press fit into the metal insert. However, the thin wall of the insert may deform outwardly to some extent in this process creating undesirable circumferential hoop stress in the ends of the resin handle. This hoop stress can shorten the life of the instrument and is to be avoided.
Increasing the thickness of the wall of the insert tends to reduce hoop stress at the expense of adding weight to the instrument. The increased weight detracts from the advantages of using resin making this a less desirable solution.
There continues to be a need for light-weight resin handles which can pass the ISO 13397-1 torque test. Preferably such instruments would have usable lives comparable to traditional metal instruments.
The present instrument is, in one embodiment, formed of an injection molded handle with a metal insert preferably hollow, through its center to provide structural reinforcement. A tapered metal cone is attached at at least one end to smoothly transition the molded handle to a smaller instrument tip. The cone may or may not have a finger pad or a band, which fits snugly in a groove located on the cone. The cone is preferably attached to the handle with adhesive.
The metal insert is placed inside a mold. Resin is then injected around the insert. The resin fills the mold to the desired shape and forms the desired handle geometry. The centrally located metal insert becomes an integral part of the structure.
To be sure that this insert will not separate from the resin, at least one spiral groove is formed on the outer surface of the insert. Preferably, two separate spirals are cut on the insert""s surface, one from each end. The opposing spirals neutralize the occurrence of any potential thermally induced axial forces.
Since the handle is repeatedly sterilized, it is conceivable that over time the insert could start to shift inside the resin in one direction. This may create an undesirable configuration as the cone will begin to separate from the main body. A double spiral configuration minimizes the likelihood of development of such unidirectional forces.
The spiral design also serves to achieve both axial and torsional interlock. Since the resin gets trapped inbetween the spiral features, the insert can neither rotate nor translate. This provides for a secure bond between the insert and the exterior handle.
A tip receiving cone for a dental instrument has a body with a hollow stem with a free end and a displaced tip receiving region. An adhesive, or glue, engaging feature is carried adjacent to the free end.
In one embodiment, the feature is an opening for example a slot or hole, formed in the stem. In another embodiment, the feature is a protrusion on the stem. The feature, combined with adjacent cured adhesive, resists larger amounts of applied torque than is achievable using only the shear strength of the adhesive.
Where the feature is a slot or hole in the stem, the cured adhesive in the slot or hole forms a physical barrier to rotation of the stem. A similar result can be obtained with a surface or member that protrudes from the stem.
The cone can be used with a relatively large diameter plastic handle that has a central hollow metal insert. The plastic handle could have an exemplary diameter on the order of 0.390 inches. The insert could have an exemplary diameter on the order of 0.219 inches.
The end or ends of the insert can be filled with adhesive. The cone can be inserted into the insert, in part with an interference fit.
The adhesive fills the stem and the areas abutting the feature, slot or protrusion. When cured, in the case of the slot, a barrier of adhesive extends through the slot blocking rotation of the cone relative to the handle.
The solid adhesive barrier in combination with the cone and metal insert resist applied torque in excess of 30 inch-pounds with a relatively light-weight metal insert. This result can be achieved with reduced interference, less of a press-fit, between the hollow end of the insert and the stem of the cone and without subjecting the plastic handle to undesirable hoop stress.
The metal cone is attached to the handle by pressing it into an open end of the metal insert inside the handle. The inside diameter of the insert is controlled at one or both ends to provide a selected press fit.
A bead of epoxy glue is used to hold the cone securely in pace once the glue has been cured. The epoxy glue is repeatedly sterilizable.
A slot, or hole, is formed at the end of the stem of the cone. As the cone is pressed in place, the glue flows into the cutout. The glue remains trapped in that location even after curing. In addition to the shear strength of the glue, the cured, hard mass of trapped glue provides a mechanical inter-lock, which significantly increases the torsional strength of the instrument.
The additional torsional strength is achieved without increasing the force needed to press fit the cone into the insert. Hoop stress, which is particularly detrimental in resin handles, can increase exponentially with an increase in press fit forces. A build up of such hoop stress at each end, although invisible when the instrument is new, can lead to premature failure of the resin when subjected to repeated sterilization. The slot or hole at the free end of the cone, helps to achieve the desired torsional strength, on the order of 400 newton-centimeters, without increasing the press-fit (the hoop stress causing mechanism) thereby maintaining the structural integrity of the instrument.
The same concept may be applied also to metal handles. In metal handles the cone is often brazed to the handle since the metals can survive the brazing temperatures. The necessary torsional strength is achieved from the brazed joint. However, this process is fairly expensive.
An alternate process would be to use epoxy glue instead of the braze paste. Providing the slot at the handle-end of the cone increases torsional strength of the instrument.
Alternate glue engaging features can be used instead of the above noted slot. For example, one or more holes can be cut into the stem of the cone. As the cone is pressed into place, the adhesive or glue flows into the hollow stem and into and through the hole or holes. When cured, the adhesive, extending through the hole or holes, blockingly engages the cone thereby preventing rotation.
Alternately, a flange or shoulder can extend radially from the stem. When inserted, the flange or shoulder protrudes into and abuts the adhesive. When cured, glue, or adhesive, located adjacent to the flange locks rotation of the cone relative to the insert.
In one aspect, the metal cone is attached to the handle by press fitting it into a glue filled metal tube that is centered in a molded resin handle. The inside diameter of this tube is controlled at each end to provide a press fit without imparting excess hoop stress to the end of resin handle.
As the cone is pressed in place in the tube, the glue flows into the cone and flows around a glue engaging feature. The glue remains trapped in that location after curing.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.